When a fluid reservoir has to be alternatively connected to a plurality of fluid lines (preferably two), fluid distribution valves are used. An example of such fluid distribution valves are the so called changeover valves. In case of changeover valves, comprising two alternate fluid line connections and a common fluid port, the changeover valves are designed in a way that a common fluid port can at least be fluidly connected to either a first fluid line connection or to a second fluid line connection. Depending on the actual design chosen, the changeover valve might also comprise a state, in which both fluid line connections are connected to the common fluid port and/or a state, in which no fluid connection is provided between any of the three fluid ports involved (i.e. the two fluid line connection ports and the common fluid port).
Changeover valves are used in a plethora of technical applications, where fluid connections have to be established and interrupted between several fluid ports. The fluids involved can be of any kind. For example, the fluid involved can be a gas, a liquid, a mixture of a gas and a liquid, a mixture of a gas and a solid (smoke), a mixture of a liquid and a solid (suspension), as well as a mixture of a gas, a liquid and a solid.
One possible technical application for the use of changeover valves are fluid working machines. Such fluid working machines comprise in particular hydraulic pumps, hydraulic motors and machines, being usable as a hydraulic pump and as a hydraulic motor. Such fluid working machines usually comprise one or several cavities of cyclically changing volume. The currently most widely used design for a cavity of cyclically changing volume is the cylinder-and-piston type design. The pumping is performed by a pumping cycle, consisting of a series of individual pumping steps. In the first pumping step, one of the pump's cavities is connected to a low pressure fluid reservoir, as soon as the pumping cavity's volume starts to expand. When the pumping cavity's volume reaches its maximum, the fluid connection between the low pressure fluid reservoir and the pumping cavity is closed and a connection between a high pressure fluid manifold and the pumping cavity is established. As the pumping cavity's volume starts to contract, the fluid, being pressurized within the pumping cavity's volume, is expelled towards the high pressure fluid manifold.
The opening and closing of the valves and/or the changing of the fluid line interconnecting states can be performed by passive means, as well as by active means. An actuation by passive means is done, if the respective valve opens and closes in response to the pressure differences of the fluid itself. Such passive valves can be used in particular for pumping purposes. For example, spring loaded check valves could be used as said passive valves.
Actively actuated valves, however, change their state according to a signal, which is externally applied. The external signal can be a mechanical signal (for example the movement of a cam, mounted on a camshaft), an electrical signal (if an electrically actuated valve is used), and/or a pressure signal (if a hydraulically actuated valve is used). Such actuated valves are used in particular for hydraulic motors and/or for more evolved hydraulic pumps.
Fluid working machines can be designed with separate (passive and/or actively controlled) fluid inlet valves and fluid outlet valves. Examples for such a design of fluid working machines are described in U.S. Pat. Nos. 5,338,160 A and 6,651,545 B2, for example. The drawback of such fluid working machines is in particular that they are using a high number of fluid valves, resulting in a relatively complex and expensive design. Furthermore, the increased number of fluid valves usually results in a more complex actuation scheme of the valves, and in relatively large mounting volume, which has to be provided for the valves.
Therefore, fluid working machines, using changeover valves for alternately connecting the cyclically changing fluid cavities to a low pressure fluid reservoir and a high pressure fluid reservoir, respectively, already have been proposed. An example of such a fluid working machine can be found in WO 2007/034301 A1. Here, the cavities, used for pumping purposes, are connected to either the fluid inlet port or the fluid outlet port by means of a changeover valve.
Another design of a changeover valve, which can be used for switching the fluid flow direction of a high pressure hydraulic medium, is described in U.S. Pat. No. 4,664,150 A.
To reduce the complexity of fluid working machines, it has been proposed in U.S. Pat. No. 7,300,260 A, to arrange the low-pressure fluid reservoir on the inside of the machine, and the high-pressure fluid manifold on the outside of the machine. This way, the lubrication of mechanical parts on the inside of the fluid working machine (such as an eccentric crank or a swash plate) can be done by the low-pressure fluid manifold. Additionally, leakage of hydraulic fluid can be dealt easily with, this way. On the other hand, the high pressure fluid usually needs to be on the outside of the machine, so that it can be used by hydraulic consumers. However, the design disclosed in U.S. Pat. No. 7,300,260 A suffers from a plurality of drawbacks. For example, the volume of the fluid channels between the pumping cavity and the valves show a high “dead volume”, causing a low efficiency of the machine. Furthermore, the use of magneto-rheological fluids is expensive and shows relatively high wear.
For minimizing the losses involved with the use of changeover valves in fluid working machines, it is desired that the cross sections of the fluid flow channels are as large as possible. In particular, the fluid flow cross section for a fluid flow between the low-pressure fluid port and the pumping chamber's fluid port, as well as the available fluid flow cross section for a fluid flow between the high-pressure fluid port and the pumping chamber's fluid port has to be as large as possible. Furthermore, the fluid flow channels, leading towards the changeover valve have to have a high cross section, as well.
However, when using changeover valves according to the state of the art together with fluid working machines it was so far not possible to achieve all these requirements at the same time. Instead, a compromise had to be chosen so far. Considering the embodiment shown in FIG. 6 of U.S. Pat. No. 6,651,545 B2, for example, it is clear from the drawing that the fluid channel connecting the pumping cavity with the changeover valve is comparatively small. Therefore, in current designs of fluid working machines the use of changeover valves for the interconnection of low pressure fluid manifolds, high pressure fluid manifolds and pumping cavities plays only a minor role, if at all.